The present invention is directed to digital printing systems that handle numerous document and data formats. Such systems are preferably open systems where devices from different manufacturers are integrated to provide solutions to customers' needs. A key feature in such printing systems is enabling electronic documents from various sources to be printed on output devices. In order to achieve device-independence efficient methods of image resolution conversion or enhancement are required. Hence, raster conversion technology, where a bitmap created for a first output device (e.g., at 240 spots per inch) is altered so as to be adequately rendered by an output device having a different resolution, is a very important aspect of the open system technology.
Digital documents created on mainframe or similar legacy computer systems may be digitized at resolutions as low as 240 spots per inch (spi). However, many modern printers operate at 300 spi or 600 spi and it is desirable to enable artifact-free printing of these 240 spi source images on 300 or 600 spi printers. While it is known to accomplish resolution conversion using bit or pixel-level conversion (e.g., bit replication), as employed in the Xerox DocuTech Production Publisher and Xerox 4850/4890 Highlight Color Printers, the present invention enables device-independent printing using an area mapping technique during image processing to enhance binary input images having a first resolution and producing images having a second resolution while reducing or eliminating image distortions upon printing.
Heretofore, resolution enhancement has been accomplished using various techniques, including those described in the following disclosures which may be relevant:
Torrey Pines Research, Behind Hewlett-Packard's Patent on Resolution Enhancement.TM. Technology, (Becky Colgan ed., BIS CAP International, 1990) pp. 1-60.
James C. Stoffel et al., A Survey of Electronic Techniques for Pictorial Image Reproduction, IEEE Transactions on Communications, Vol. COM-29, No. 12, December 1981.
L. Steidel, Technology Overview: Resolution Enhancement Technologies for Laser Printers, LaserMaster Corp.
U.S. Pat. No. 4,437,122 to Walsh et al., issued Mar. 13, 1984, teaches an improved method of converting low resolution images into images of higher resolution for printing so as to simultaneously increase density and smooth character edges. In a CRT display or hardcopy output apparatus, the invention is accomplished by converting an original pixel into a higher resolution 3.times.3 enhanced representation. The status of each of the nine elements in the enhanced representation is determined as a result of an examination of the neighboring pixels of the original pixel.
U.S. Pat. No. 4,841,375 to Nakajima et al., issued Jun. 20, 1989, discloses an image resolution conversion apparatus that converts image data having a predetermined pixel density to a pixel density matching that of a printer so as to enable printing by the printer. The pixel density converter includes: a conversion-pixel position detector for detecting the position of a converted pixel; an original-pixel extractor for extracting a reference original pixel; a conversion-pixel density operation circuit for calculating the density of a conversion pixel; a threshold-value setter for dynamically setting a threshold value; a binary encoding circuit for digitizing the conversion-image density; an input interface for inputting image data; an output interface for outputting image data; and a control circuit for controlling the input/output (I/O) and the conversion operations.
U.S. Pat. 4,847,641 (issued Jul. 11, 1989) and U.S. Pat. No. 5,005,139 (issued Apr. 2, 1991) to Tung disclose print enhancement circuitry for a laser beam printer. The bit map of a region of the image to be output is compared to a number of patterns or templates. When a match is detected, a section of the bitmap that was matched is replaced with a unique bitmap section designed to compensate for errors. The replacement bitmap section may include predetermined shifting of some dot positions to compensate for the error in the original bitmap section.
U.S. Pat. No. 5,539,866 (issued Jul. 23, 1996) to Banton et al. teaches increasing the resolution of a binary input image to be printed by an electronic printing system. The method includes storing a portion of the binary input image and determining the binary state of a central pixel and a plurality of pixels neighboring the central pixel and comparing the states of the central pixel and the neighboring pixels to a predefined template. Based upon the comparison, the method detects when a match occurs a multi-bit digital signal is generated as a value associated with the predefined template and the multi-bit digital signal is converted into a resolution enhanced output pixel for printing by the system.
U.S. Pat. application Ser. No. 07/513,415, and the corresponding Japanese laid-open patent publication 4-227584 published Aug. 17, 1992, to Mailloux et al. disclose a method to enable the conversion of binary image data originally generated at a lower resolution into representative binary image data of a higher resolution, wherein the conversion ratio, or magnification factor, is an integer value. Included within the resolution magnification invention are methods for smoothing the interpolated output image and thereby reducing objectionable visual characteristics.
The following pending patent applications describe image resolution conversion methods employing morphological processes: appl. No. 08/169,487 by Loce et al. (filed Dec. 17, 1993) and corresponding Japanese Patent Appl. No. 7,226,884 on Aug. 22, 1995, and appln. No. 08/451,376 (filed May 23, 1995), both of which are assigned to the assignee of the present application.
Publication by Edward Dougherty et al. "Optimal binary differencing filters: design, logic complexity, precision analysis, and application to digital document processing," Journal of Electronic Imaging, Jan. 1996. Vol 5, no. 1, p 66-86, discloses statistical and logical properties of template-matching filters. Also disclosed are design methods for optimal template matching filters for digital document restoration and resolution conversion and enhancement.
In accordance with the present invention, there is provided a method for processing a highlight color digital input image signal representing a highlight color image, comprising the steps of: quantization slicing the input image signal to produce at least a first binary image and a second binary image; passing, in parallel channels, each of the first and the second binary images through a binary, template-based resolution conversion operation to yield an intermediate image, wherein the intermediate image is a high-addressability image; arbitrating between the intermediate image available on the parallel channels and associating a marker with each pixel of the intermediate image, said marker indicating the result of the arbitrating step; passing the intermediate image and associated markers to a lookup table, wherein the lookup table outputs, in response to the image pixel and associated marker a digital signal suitable for driving a pulse-width, position modulated marking engine.
One aspect of the invention is based on the discovery of a new technique for improving the image quality of resolution enhanced images, and in particular highlight color images. The techniques of the present invention are intended to enable the processing of highlight color images wherein the techniques increase the resolution of an input image to a common resolution of a highlight color output device while preserving local average density, stroke or line width and edge position in the output image.
Such techniques can be implemented, for example, by an image processor working in conjunction with a highlight color printing system. For example, a machine implementing the invention can include the Xerox.RTM. 4850 Laser Printing System or 4890 Highlight Laser Printer.
One aspect of the invention is based on the observation of problems with conventional resolution conversion techniques, particularly techniques that result in strong halftone moire when converting from 240 spi input to 300 spi output. This aspect is based on the discovery of a technique that alleviates these problems by employing a computationally simple, moire-free conversion. This technique can be implemented, for example, using interlocked area mapping techniques and pixel averaging in an image processing architecture designed to provide highlight color image processing.
The techniques described herein are advantageous because they are computationally efficient, simple and do not require modifications to the output device to achieve the improved image quality. The present invention, therefore, may be adapted to any of a number of output devices where open systems interfaces require the receipt and rendering of images having various resolutions.